Transformer with integrated inductor

ABSTRACT

The present invention relates to a transformer with an additional integrated inductor. To this end it is envisaged that the transformer is provided with a first core ( 1 ) which has at least a primary winding ( 5 ), a second core ( 2 ) which has at least a secondary winding ( 4 ), and a third core ( 3 ), while each core ( 1, 2, 3 ) is a separate component.

[0001] The present invention relates to a transformer with an additional integrated inductor and to a switched-mode power supply comprising such a transformer.

[0002] Switched-mode power supplies require at least one inductive component and one capacitive component for the resonant circuit. The inductive component is usually a coil (inductor) which along with a capacitor is connected to the primary or secondary winding of a transformer. Such a switched-mode power supply with a resonant construction is known from U.S. Pat. No. 4,692,851. Here the primary winding is connected in series with an inductor, whereas on the secondary side the main secondary winding has a parallel capacitance. In this way the inductance of the inductor along with the capacitance of the capacitor provide the necessary resonant oscillations.

[0003] It is an object of the present invention to provide a transformer with an integrated stray inductance so that a separate inductive component can be dispensed with and a simplification of the design of resonant switched-mode power supplies is possible.

[0004] The object is achieved in accordance with the invention in that a transformer is provided with a first core which has at least a primary winding, a second core which has at least a secondary winding, and a third core, while each core is a separate component.

[0005] Through the structural separation of the individual cores it is possible to optimize each of these in relation to the main flux or the stray flux. Nevertheless, standard components can be used, because each core only has one section. This reduces the manufacturing costs considerably. Since the main flux is dependent upon the so-called main permeance and the stray flux on the so-called stray permeance, these permeances must be selected accordingly. The permeances are in turn dependent upon the respective air gap between the individual cores which has a certain cross-sectional area and a certain length. The cores can, however, be bonded to each other when only little permeance is necessary.

[0006] The embodiments as claimed in claims 2 and 3 offer the advantage that the three cores can be optimally matched to the flux that they carry. Thus both the main flux and the stray flux may be determined individually. In addition, the stray permeance and the main permeance can be determined via the air gap between the individual cores.

[0007] The embodiment as claimed in claim 4 offers the major advantage that an additional coil (inductor) in series with either the primary winding or the secondary winding is not necessary. Since resonant switched-mode power supplies require such a coil, their structure is thus simplified since one component can be spared.

[0008] The embodiments as claimed in claims 5 to 8 represent particularly advantageous embodiments of the individual cores.

[0009] The embodiment as claimed in claim 9 allows a particularly low-cost design of a transformer in accordance with the invention. The manufacturer of magnet cores for transformers offers all standard sections. Magnet cores with other sections are considerably more expensive which in mass-production is a factor that cannot be underestimated. It is therefore sensible to use standard components for the three magnet cores of different dimensions.

[0010] With the embodiment as claimed in claim 10, the advantage of the variable stray inductance of a transformer in a switched-mode power supply is used. This is preferably a resonant switched-mode power supply.

[0011] The present invention is described in more detail using an example of embodiment and various Figures, in which:

[0012]FIG. 1 shows an equivalent circuit diagram of the magnetic circuit in a transformer in accordance with the invention and

[0013]FIG. 2 shows a side view of a transformer in accordance with the invention.

[0014] A transformer with integrated inductor consists of an initial iron core 1, the core for the primary winding 5, a second iron core 2, the core for the secondary winding 4, and a third iron core, the stray flux core 3. All three iron cores 1, 2, 3 are separate components and are therefore structurally separated from each other by air gaps 6, 7. Thus the sections of the individual cores can be dimensioned fully independently of each other. The first core 1 thereby carries the primary winding 5, while the second core 2 carries the secondary winding 4. The arrangement is designed in such a way that the first U-shaped core 1 with its open side is only separated from the second I-shaped core 2 by an air gap 6. This I-shaped core 2 in turn borders on a third also U-shaped core 3, separated by an air gap 7, which core 3 faces the second core 2 with the open side and represents the stray flux core. The cores are thus arranged in a row, as can also be seen from FIG. 2. Since cores 1, 2 and 3 do not touch, they are mounted on a joint component in a manner that is not shown here. This component may be a printed circuit board, as is customary in switched-mode power supplies. In addition, at least the air gap 6 between the first core 1 and the second core 2 can be dispensed with and in its stead another medium may be present. In particular the first core 1 may also be secured to the second core 2 by bonding or some other means.

[0015] On the primary winding 5 with N₁ windings a magnetic potential F₁=N₁*I₁ is applied while on the secondary winding 4 with N₂ windings a corresponding magnetic potential F₂=N₂*I₂ is applied. The so-termed main permeance P_(h), which determines the main inductance is realized here by the air gaps 6. These air gaps 6 each have a section A_(h), which is identical with section A₁ of the first core 1 and a length 1 _(h), which corresponds to the distance between the first core 1 and the second core 2. The central I-shaped core 2 accordingly has the section A₂. The so-termed stray permeance P_(s), which determines the stray inductance, is realized by the air gaps 7. These air gaps 7 each have a section A_(s), which is identical with the section A₃ of the third core 3, and a length I_(s), which corresponds to the distance between the third core 3 and the second core 2. Since the air gaps 6 and 7 are both present twice, the resultant permeances are P_(h)=μ_(o)*A_(h)/2l_(h) for the main permeance P_(h) and P_(s)=μ_(o)*A_(s)/2l_(s) for the stray permeance P_(s). The relationship with the currents I₁ and I₂ can be established from the two permeances and the magnetic resultant flows Φ_(h) and Φ_(s) in accordance with the magnetic equivalent circuit diagram in FIG. 1, because it holds that Φ_(h)=(F1−F2)*Ph and Φ_(s)=F₂*P_(s). With the arrangement in accordance with the invention the lengths l_(h) and l_(s) and the sections A₂, A_(h) (=A₁) and A_(s) (=A₃) can be selected independently of one another. This gives the constructor more freedom of design and allows optimum adaptation of the sections A₁, A₂ and A₃ of the cores 1, 2, 3 to the respective resultant magnetic flux to be conveyed.

[0016] The permeances as well and thus the inductances can be freely selected over the lengths l_(h) and l_(s). In spite of this standard sections can also be used here to reduce costs, if each core has its own standard section. The forming of cores 1 and 3 into a U-shape and core 2 into an I-shape is of course not the only conceivable solution. In particular, the legs of the two U-shaped cores 1 and 3 can be made longer or shorter. It is also possible to arrange core 1 or core 3 relative to core 2 offset at an angle of for example 90°. In this way the length of the arrangement is shortened, so that then a larger width is needed. Overall, however, this makes the transformer more compact. 

1. A transformer provided with a first core (1) which has at least a primary winding (5), a second core (2) which has at least a secondary winding (4), and a third core (3), while each core (1, 2, 3) is a separate component.
 2. A transformer as claimed in claim 1, characterized in that the first core (1) is separated from the second core (2) by at least an air gap (6) and the second core (2) is separated from the third core (3) by at least a further air gap (7).
 3. A transformer as claimed in claim 2, characterized in that the air gaps (6, 7) are designed so that the first core (1) essentially carries a main flux (Φ_(h)), in that the third core (3) essentially carries a stray flux (Φ_(s)) and in that the second core essentially carries the difference between the main flux and the stray flux.
 4. A transformer as claimed in claim 3, characterized in that the stray flux (Φ_(s)) through the third core (3) has the effect of an inductor integrated with the transformer, which inductor is connected in series with the secondary winding (4).
 5. A transformer as claimed in any one of claims 1 to 4, characterized in that the first core (1) has a U-shaped design.
 6. A transformer as claimed in any one of claims 1 to 5, characterized in that the third core (3) has a U-shaped design.
 7. A transformer as claimed in any one of claims 1 to 6, characterized in that the second core (2) has an I-shaped design.
 8. A transformer as claimed in any one of claims 1 to 7, characterized in that he first core (1), the second core (2) and the third core (3) each have different sections (A₁, A₂, A₃).
 9. A transformer as claimed in any one of claims 1 to 8, characterized in that the first core (1), the second core (2) and the third core (3) each have a standardized section (A₁, A₂, A₃).
 10. A switched-mode power supply with a transformer provided with a first core (1) which has at least a primary winding (5), a second core (2) which has at least a secondary winding (4), and a third core (3), while each core (1, 2, 3) is a separate component. 